Until recently, musicians seeking relief had only limited opportunity for expert care. But methods of relaxation and balancing, such as yoga, tai chi and the very popular Alexander technique are offered in most cities and at many festivals. And some solo practitioners have developed impressive reputations for healing. In an apartment in New York's Carnegie Hall, a Lithuanian-born physical therapist named Shmuel Tatz, who bills himself as a "body tuner," has garnered endorsements from such celebrity musicians as Isaac Stern, Yehudi Menuhin, Rosalyn Tureck and Christa Ludwig. A session with Mr. Tatz, who prods and massages and puts you through a rigorous routine of movement and posture adjustment, does amount to a tuning of sorts, and his admirers are legion.